The development of the vascular system (sometimes referred to as the vascular tree) involves two major processes: vasculogenesis and angiogenesis. Vasculogenesis is the process by which the major embryonic blood vessels originally develop from early differentiating endothelial cells such as angioblasts and hematopoietic precursor cells that in turn arise from the mesoderm. Angiogenesis is the term used to refer to the formation of the rest of the vascular system that results from vascular sprouting from the pre-existing vessels formed during vasculogenesis (see, e.g., Risau et al. (1988) Devel. Biol., 125:441–450). Both processes are important in a variety of cellular growth processes including developmental growth, tissue regeneration and tumor growth, as all these processes require blood flow for the delivery of necessary nutrients.
Thus, angiogenesis plays a critical role in a wide variety of fundamental physiological processes in the normal individual including embryogenesis, somatic growth, and differentiation of the nervous system. In the female reproductive system, angiogenesis occurs in the follicle during its development, in the corpus luteum following ovulation and in the placenta to establish and maintain pregnancy. Angiogenesis additionally occurs as part of the body's repair processes, such as in the healing of wounds and fractures. Thus, promotion of angiogenesis can be useful in situations in which establishment or extension of vascularization is desirable. Angiogenesis, however, is also a critical factor in a number of pathological processes, perhaps must notably tumor growth and metastasis, as tumors require continuous stimulation of new capillary blood vessels in order to grow. Other pathological processes affected by angiogenesis include conditions associated with blood vessel proliferation, especially in the capillaries, such as diabetic retinopathy, arthropathies, psoriasis and rheumatoid arthritis.
Given its key role in both normal physiological and pathological processes, not surprisingly considerable research effort has been directed towards identifying factors involved in the stimulation and regulation of angiogenesis. A number of growth factors have been purified and characterized. Such factors include fibroblast growth factors (FGFs), platelet-derived growth factor (PDGF), transforming growth factor alpha (TGFα), and hepatocyte growth factor (HGF) (for reviews of angiogenesis regulators, see, e.g., Klagsbrun et al. (1991) Ann. Rev. Physiol., 53:217–39; and Folkman et al. (1992) J. Biol. Chem., 267:10931–934).
Current research indicates that a family of endothelial cell-specific growth factors, the vascular endothelial growth factors (VEGFs), together with their cognate receptors, are primarily responsible for stimulation of endothelial cell growth and differentiation. These factors are members of the PDGF family and appear to act primarily via endothelial receptor tyrosine kinases (RTKs).
The first identified and most well studied member of this particular family is the vascular endothelial growth factor (VEGF), also referred to as VEGF-A. This particular growth factor is a dimeric glycoprotein in which the two 23 kD subunits are joined via a disulfide bond. Five VEGF-A isoforms encoded by distinct mRNA splice variants appear to be equally effective in stimulating mitogenesis in endothelial cells, but tend to have differing affinities for cell surface proteoglycans.
VEGF-A acts to regulate the generation of new blood vessels during embryonic vasculogenesis and then subsequently plays an important role in regulating angiogenesis later in life. Studies showing that inactivation of a single VEGF-A allele results in embryonic lethality provide evidence as to the significant role this protein has in vascular development and angiogenesis (see, e.g., Carmeliet et al. (1996) Nature 380: 435–439; and Ferrara et al. (1996) Nature, 380: 439–442). VEGF-A has also been shown to have other activities including a strong chemoattractant activity towards monocytes, the ability to induce the plasminogen activator and the plasminogen activator inhibitor in endothelial cells, and to induce microvascular permeability. VEGF-A is sometimes also referred to as vascular permeability factor (VPF) in view of this latter activity. The isolation and properties of VEGF-A have been reviewed (see, e.g., Ferrara et al. (1991) J. Cellular Biochem. 47: 211–218; and Connolly, J. (1991) J. Cellular Biochem. 47:219–223).
Alternative mRNA splicing of a single VEGF-A gene gives rise to at least five isoforms of VEGF-A. These isoforms are referred to as VEGF-A121; VEGF-A145; VEGF-A165; VEGF-A189; and VEGF-A206. As the name implies, the VEGF-A165 isoform is a 165 amino acid species and has a molecular weight of approximately 46 kD; this isoform is the predominant molecular form found in normal cells and tissues. VEGF-A165 includes a 44 amino acid region near the carboxyl-terminal region that is enriched in basic amino acid residues. It also exhibits an affinity for heparin and heparin sulfates.
VEGF-A121 is the shortest form, with a deletion of 44 amino acids between positions 116 and 159 as compared to the VEGF A165 isoform. It is freely diffusible in the surrounding extracellular matrix. VEGF-A189 is a longer form with an insertion of 24 highly basic residues at position 116 with respect to VEGF A165. The VEGF-A206 isoform includes insertion of 41 amino acids with respect to the VEGF A165 isoform, including the 24 amino acid insertion found in VEGF-A189. VEGF-A121 and VEGF-A165 are soluble proteins, with VEGF-A165 being the predominant isoform secreted by cells. In contrast, VEGF-A189 and VEGF-A206 appear to be mostly cell-associated. All of these isoforms of VEGF-A are biologically active.
VEGF-B, also referred to as VRF, has similar angiogenic and other properties to those of VEGF-A, but is distributed and expressed in tissues differently from VEGF-A. In particular, VEGF-B is very strongly expressed in heart, and only weakly in lung, whereas the reverse is the case for VEGF-A. This suggests that VEGF-A and VEGF-B, despite the fact that they are co-expressed in many tissues, may have functional differences. The amino acid sequence of VEGF-B is approximately 44% identical to that of VEGF-A. Alternative exon splicing of the VEGF-B gene generates two isoforms encoding human proteins of 167 and 186 amino acids, and referred to as VEGF-B167 and VEGF-B186, respectively. VEGF-B167 tends to remain cell-associated, while VEGF-B186 is freely secreted. The isolation and characteristics of these isoforms are discussed in PCT/US96/02957 and in Olofsson et al. (1996) Proc. Natl. Acad. Sci. USA 93: 2576–2581.
VEGF-C is also referred to as VEGF-related protein (hence VRP) or VEGF-2. The protein is roughly 30% identical to the amino acid sequence of VEGF-A, and includes N-terminal and C-terminal extensions not present in VEGF-A, VEGF-B or P1GF (infra). Although the protein induces vascular permeability and promotes endothelial growth, it is less potent than VEGF-A. Its isolation and characteristics are disclosed in Joukov et al. (1996) EMBO J. 15: 290–298.
VEGF-D was isolated from a human breast cDNA library, commercially available from Clontech, by screening with an expressed sequence tag obtained from a human cDNA library designated “Soares Breast 3NbHBst” as a hybridization probe (Achen et al. (1998) Proc. Natl. Acad. Sci. USA 95:549–553). The protein is also referred to as FIGF, for c-fos-induced growth factor. The VEGF-D gene is expressed most abundantly in lung, heart, small intestine and fetal lung. It is found at lower levels in skeletal muscle, colon and pancreas. Its isolation and characteristics are discussed in PCT Publication WO 98/07832.
Recently several additional VEGF-like proteins have been identified from various strains of orf viruses; in the literature these viral VEGF proteins have sometimes been collectively referred to as VEGF-E. One protein, variously referred to as OV NZ2, ORFV2-VEGF, OV-VEGF2, and VEGF-ENZ2 has been isolated from the orf viral strain NZ2 (see, e.g., Lyttle, D. J. et al. (1994) J. Virology 68:84–92; and PCT Publication WO 00/25805). Another viral VEGF-like protein referred to as NZ7, OV-VEGF7, VEGF-E and VEGF-ENZ7 has been found in the NZ7 strain of orf viruses. This protein exhibits potent mitogenic activity but lacks the basic domain of certain VEGF proteins such as VEGF-A165 (see, e.g., Lyttle, D. J. et al. (1994) J. Virology 68:84–92; and Ogawa, S. et al. (1998) J. Biol. Chem. 273:31273–31282). A third VEGF-like protein has been identified in a NZ strain, specifically a NZ10 strain and is referred to simply as NZ10 (see, e.g., PCT Publication WO 00/25805). Yet another VEGF-like protein has been identified in the orf virus strain D1701 and in some instances has been referred to as VEGF-ED1701 (see, e.g., Meyer, M. et al. (1999) EMBO J. 18:363–74).
In addition to these viral VEGF-E genes, a VEGF-like growth factor isolated from mammalian sources has also been named VEGF-E. The isolation and characterization of this VEGF-like factor is discussed in PCT Publication WO 99/47677.
Another VEGF-like protein has been termed PDGF/VEGF-Like Growth Factor H, or simply VEGF-H. It is discussed in PCT Publication WO 00/44903. Additional VEGF-like proteins include one called VEGF-R (see, e.g., PCT Publication WO 99/37671) and another referred to as VEGF-X (see, e.g., PCT publication WO 00/37641). Most recently a VEGF protein referred to as VEGF-138 has been identified by Neufeld and others. A final protein related to the VEGF proteins is the Placenta Growth Factor, P1GF. This protein was isolated from a term placenta cDNA library. A segment of the protein exhibits high levels of homology with VEGF-A. Its isolation and characteristics are described by Maglione et al. (1991) Proc. Natl. Acad. Sci. USA 88:9267–9271. Its biological function is presently not well understood. Two alternatively transcribed mRNAs have been identified in humans (P1GF-1 and P1GF-2).
The foregoing PDGF/VEGF family members act primarily by binding to receptor tyrosine kinases. Five endothelial cell-specific receptor tyrosine kinases have been identified thus far, namely VEGFR-1 (also called Flt-1), VEGFR-2 (also called KDR/Flk-1), VEGFR-3 (Flt4), Tie and Tek/Tie-2. Each of these kinases have the tyrosine kinase activity necessary for signal transduction. The essential, specific role in vasculogenesis and angiogenesis of VEGFR-1, VEGFR-2, VEGFR-3, Tie and Tek/Tie-2 has been demonstrated by targeted mutations inactivating these receptors in mouse embryos.
VEGF-A binds VEGFR-1 and VEGFR-2 with high affinity, as well as neurophilin 1. As just indicated, VEGFR-1 binds VEGF-A, but also binds VEGF-B and P1GF. VEGF-C has been shown to be the ligand for VEGFR-3, and it also activates VEGFR-2 (Joukov et al. (1996) The EMBO Journal 15: 290–298). Both VEGFR-2 and VEGFR-3 bind VEGF-D. Initial studies with the viral VEGF proteins (i.e., the viral VEGF-E group) show that these proteins selectively bind VEGFR-2 but not VEGFR-1 (see, e.g., Ogawa, S. et al. (1998) J. Biol. Chem. 273:31273–31282; and Meyer, M. et al. (1999) 18:363–74). A ligand for Tek/Tie-2 has been described in PCT Publication WO 96/11269. The ligand for Tie has not yet been identified. Additional details regarding the various VEGF receptors are provided in PCT Publication WO 00/25805.
Recently, a 130–135 kDa VEGF isoform specific receptor has been purified and cloned (Soker et al. (1998) Cell 92:735–745). The evidence indicates that this VEGF receptor specifically binds to the VEGF165 isoform via the exon 7 encoded sequence of VEGF165, which sequence shows weak affinity for heparin (Soker et al. (1998) Cell, 92:735–745). The receptor has also been found to be identical to human neurophilin-1 (NP-1), a receptor involved in early stage neuromorphogenesis. One of the splice variants of P1GF, namely P1GF-2, also appears to interact with NP-1 (Migdal et al., (1998) J. Biol. Chem. 273: 22272–22278).
Thus, a variety of cell growth factors, in particular VEGF proteins and VEGF-related proteins, have been identified. Certain receptors that bind to the VEGF proteins have also been identified. However, modulation of the expression of VEGF proteins and VEGF-related proteins so as to modulate the process of angiogenesis has not been described. The ability to modulate the process of angiogenesis in a cell or group of cells, using one or more exogenous molecules, would have utility in activating beneficial aspects associated with endothelial cell growth and in repressing non-beneficial aspects.